1. Field of the Invention
The present invention relates generally to an air-fuel ratio control system for an internal combustion engine, and more specifically, to the air-fuel ratio control system which feedback controls a fuel injection amount so as to control an air-fuel ratio monitored based on the exhaust gas downstream of a catalytic converter to a target air-fuel ratio.
2. Description of the Prior Art
As is known, a fuel injection amount for an internal combustion engine is feedback controlled so as to converge an actual air-fuel ratio monitored based on the exhaust gas to a stoichiometric air-fuel ratio in an effort to ensure the maximum purification efficiency of a catalytic converter. Further, it has been considered desirable to execute such a feedback control based on an air-fuel ratio monitored on the downstream side of the catalytic converter (hereinafter referred to as "downstream-side air-fuel ratio") since the downstream-side air-fuel ratio highly reflects a storage condition or an adsorption condition of the catalytic converter.
This type of an air-fuel ratio control system for an internal combustion engine is disclosed, such as, in Japanese First (unexamined) Patent Publication (Kokai) No. 3-185244 which corresponds to U.S. Pat. No. 5,090,199.
In the disclosed system, an air-fuel ratio sensor (hereinafter referred to as "A/F sensor") monitors an air-fuel ratio based on the exhaust gas upstream of the catalytic converter (hereinafter referred to as "upstream-side air-fuel ratio"), while an O.sub.2 sensor detects whether a downstream-side air-fuel ratio is rich or lean relative to the stoichiometric air-fuel ratio. The system performs a so-called modern control, wherein a dynamic model which is an approximation to a controlled object representing an operation series or sequence from a fuel injection valve to the A/F sensor is used to perform a state-feedback control of the fuel injection amount. Specifically, the system uses a detection value of the A/F sensor in the modern control to derive the fuel injection amount by performing the state-feedback control In such a manner as to control the upstream-side air-fuel ratio to the target air-fuel ratio. On the other hand, a detection value of the 02 sensor is used to feedback control the target air-fuel ratio used in the modern control so as to correct the target air-fuel ratio in a direction opposite to a direction of deviation of the downstream-side air-fuel ratio with respect to the stoichiometric air-fuel ratio. Accordingly, the monitored downstream-side air-fuel ratio is reflected on the state-feedback control in the form of correcting the target air-fuel ratio, and thus the control as a whole is executed to control the downstream-side air-fuel ratio to the stoichiometric air-fuel ratio.
As described above, in the conventional air-fuel ratio control system for the internal combustion engine, the downstream-side air-fuel ratio is controlled to be converged to the stoichiometric air-fuel ratio as a result of correcting the target air-fuel ratio used in the state-feedback control depending on a magnitude of the downstream-side air-fuel ratio. Accordingly, when the downstream-side air-fuel ratio is disturbed, the correction of the fuel injection mount based on the sate-feedback control in the modem control is performed only after the correction of the target air-fuel ratio based on the known normal feedback control using the detection value of the O.sub.2 sensor, i.e. the downstream-side air-fuel ratio. As a result, a time period required for converging the disturbed downstream-side air-fuel ratio to the stoichiometric air-fuel ratio is prolonged so that an improvement is necessary in view of a response characteristic of the air-fuel ratio control.